Recently, a research on a surface plasmon resonance (SPR) sensor which has relatively high sensitivity (to about 1 pg/mm2) to an object to be detected and does not need a labeling process with a phosphorescent dye and by which a degree of a reaction can be monitored in real time has been carried out as a core technology in the fields of nano chemistry and biosensors.
A surface plasmon is a collective charge density oscillation occurring on a surface of a metallic thin film, and a surface plasmon wave (SPW) generated by the oscillation is a surface electromagnetic wave propagating along a boundary surface between metal and a dielectric material. If an external electric field is applied to an interface between two media having different dielectric functions, i.e., an interface between metal and a dielectric material, surface charges are induced at the an interface between the two media due to discontinuity of vertical components of the electric field and oscillation of the surface charges is represented as a surface plasmon wave.
An incident wave of a light emitted from a light source is reflected at the interface with the metallic thin film and an evanescent wave is exponentially decreased at the interface into the metallic thin film. At a specific incident angle and with a specific thickness of the metallic thin film, an incident wave parallel to a direction of the interface and a surface plasmon wave propagating along the interface between the metallic thin film and the dielectric material have the same phase, and a resonance thus occurs. In this case, all photoenergy of the incident wave is absorbed by the metallic thin film and a reflection wave thus disappears, which is referred to as a surface plasma resonance (SPR). Further, an angle of a minimum reflectivity of the incident light is referred to as a surface plasmon resonance angle.
As a structure or an environment of the dielectric material in contact with the surface of the metallic thin film is varied, an effective refractive index is varied and the resonance angle at which a surface plasmon resonance occurs, i.e., an angle of a minimum reflectivity, is thus varied. In this way, by using a principle of a surface plasmon resonance capable of optically measuring an environmental change in a material, it is possible to detect changes such as a selective binding or separation between various kinds of materials from a change in a resonance angle via an appropriate chemical or physical change on a surface layer of the metallic thin film.
A phenomenon of a surface plasmon resonance was first described in the early 1900s by Wood as a result of observation of anomalous diffraction due to excitation of surface plasma wave in Fano's metal diffraction lattice, and in 1968, Kretschmann and Otto demonstrated excitation of two surface plasmon resonances having different structures using prisms, showing general applicability of a surface plasmon resonance. Further, in 1970s, availability of a surface plasmon resonance for analyzing characteristics of a thin film and observing a change in a metallic interface was proved, and in 1982, Nylander and Liedberg used a surface plasmon resonance sensor for detecting a gas and as a bio sensor.
However, a conventional surface plasmon resonance (SPR) sensor has a problem of low sensitivity in analyzing a material of low concentration.